# C++ named requirements: LegacyBidirectionalIterator

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< cpp‎ | named req

A LegacyBidirectionalIterator is a LegacyForwardIterator that can be moved in both directions (i.e. incremented and decremented).

If a LegacyBidirectionalIterator it originates from a Container, then it's `value_type` is the same as the container's, so dereferencing (*it) obtains the container's `value_type`.

### Requirements

The type `It` satisfies LegacyBidirectionalIterator if

- The type `It` satisfies LegacyForwardIterator

And, given

- a and b, lvalues of type `It`
- `reference`, the type denoted by std::iterator_traits<It>::reference

The following expressions must be valid and have their specified effects:

Expression | Return | Equivalent expression | Notes
--- | --- | --- | ---
--a | `It&` |  | Preconditions: a is decrementable (there exists such b that a == ++b) Postconditions: a is dereferenceable --(++a) == a If --a == --b then a == b a and --a designate the same iterator object
a-- | convertible to const It& | It temp = a; --a; return temp; |
*a-- | `reference` |  |

A mutable LegacyBidirectionalIterator is a LegacyBidirectionalIterator that additionally satisfies the LegacyOutputIterator requirements.

### Notes

The begin iterator is not decrementable and the behavior is undefined if --container.begin() is evaluated.

A bidirectional iterator does not have to be dereferenceable to be decrementable (in particular, the end iterator is not dereferenceable but is decrementable).

### Concept For the definition of std::iterator_traits, the following exposition-only concept is defined.
template<class I> concept __LegacyBidirectionalIterator = __LegacyForwardIterator<I> && requires(I i) { { --i } -> std::same_as<I&>; { i-- } -> std::convertible_to<const I&>; { *i-- } -> std::same_as<std::iter_reference_t<I>>; }; |  |
where the exposition-only concept `__LegacyForwardIterator` is described in LegacyForwardIterator. | (since C++20)

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 299 (N3066) | C++98 | the return type of *a-- was required to be convertible to `T` | changed the return type to `reference`[1]
LWG 383 | C++98 | b was required to be dereferenceable after --a | a is required to be dereferenceable instead
LWG 1212 (N3066) | C++98 | the return type of *a-- did not match the return type of *a++ required by LegacyForwardIterator | changed the return type to `reference`

1. ↑ This issue was initially resolved by N2758 (iterator concepts), which was dropped later from the C++ standard.

### See also

bidirectional_iterator (C++20) | specifies that a `forward_iterator` is a bidirectional iterator, supporting movement backwards (concept)
Iterator library | provides definitions for iterators, iterator traits, adaptors, and utility functions

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